When finely divided or pulverized coal is combusted at high temperatures, for example, in boilers for the steam generation of electricity, the ash, consisting of the incombustible residue plus a small amount of residual combustible matter, is made up of two fractions, a bottom ash recovered from the furnace or boiler in the form of a slag-like material and a fly ash which remains suspended in the flue gases from the combustion until separated therefrom by known separation techniques, such as electrostatic precipitation. This fly ash is an extremely finely divided material generally in the form of spherical bead-like particles, with at least 70 percent by weight passing a 200 mesh sieve, and has a generally glassy state resulting from fusion or sintering during combustion. As recognized in the American Society of Testing Materials (ASTM) specification designations C618-00 entitled “Fly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Portland Cement Concrete” and D5370-96 entitled “Standard Specification for Pozzolanic Blended Materials in Construction Application,” fly ash is subdivided into two distinct classifications; namely, Class-F and Class-C. The definitions of these two classes given in the aforementioned ASTM specifications are as follows:
“Class-F—Fly ash normally produced from burning anthracite or bituminous coal that meets the applicable requirements for this class as given herein. This class fly ash has pozzolanic properties.
Class-C—Fly ash normally produced from lignite or subbituminous coal that meets the applicable requirements for this class as given herein. This class of fly ash, in addition to having pozzolanic properties, also has some cementitious properties. Some Class-C fly ashes may contain lime contents higher than 10 percent.”
The latter reference to “pozzolanic properties” refers to the capability of certain mixtures that are not in themselves cementitious, but are capable of undergoing a cementitious reaction when mixed with calcium hydroxide in the presence of water. Class-C fly ash possesses direct cementitious properties as well as pozzolanic properties. ASTM C618-00 is also applicable to natural pozzolanic materials that are separately classified as Class N.
As the above quotation from the ASTM specification indicates, the type of coal combusted generally determines which class fly ash results, and the type of coal in turn is often dependent upon its geographic origin. Thus, Class-C fly ash frequently results from the combustion of coals mined in the Midwest United States; whereas Class-F fly ash often comes from combustion of coals mined in the Appalachian region of the United States. The ASTM specification imposes certain chemical and physical requirements upon the respective fly ash classifications which are set forth in U.S. Pat. No. 5,520,730, the disclosure of which is incorporated herein by reference.
Blast furnace slag is a by-product of the production of iron in a blast furnace; silicon, calcium, aluminum, magnesium and oxygen are the major elemental components of slag. Blast furnace slags include air-cooled slag resulting from solidification of molten blast furnace slag under atmospheric conditions; granulated blast furnace slag, a glassy granular material formed when molten blast furnace slag is rapidly chilled as by immersion in water; and pelletized blast furnace slag produced by passing molten slag over a vibrating feed plate where it is expanded and cooled by water sprays, whence it passes onto a rotating drum from which it is dispatched into the air where it rapidly solidifies to spherical pellets. In general, the glass content of the slag determines the cementitious character. Rapidly cooled slags have a higher glass content and are cementitious; slowly cooled slags are non-glassy and crystalline and, thus do not have significant cementitious properties.
The quantities of these by-product materials that are produced annually are enormous and are likely only to increase in the future. As petroleum oil as the fuel for the generation of electricity is reduced because of conservation efforts and unfavorable economics, and as political considerations increasingly preclude the construction of new nuclear power electrical generating facilities, or even the operation or continued operation of already completed units of this type, greater reliance will necessarily fall on coal as the fuel for generating electricity. As of 1979, the volume of Class-F fly ash that was available then was estimated to be about five times what could be readily utilized. The estimated annual total production of coal ash in the U.S. is about 66.8 million tons, while the annual total coal ash sales in the U.S. is only about 14.5 million tons. Further, in Canada, the recovery of copper, nickel, lead and zinc from their ores produces over twelve million tons of slag per year, which is usually accumulated near the smelters with no significant use. Obviously, there is an urgent and growing need to find effective ways of employing these unavoidable industrial by-products since they will otherwise collect at a staggering rate and create crucial concerns regarding their adverse environmental effects.
Various proposals have already been made for utilizing both types of fly ash. According to Lea (1971), The Chemistry of Cement and Concrete, Chemical Publishing Company, Inc., page 421 et seq., fly ash, i.e., Class-F type, from boilers was first reported to be potentially useful as a partial replacement for Portland cement in concrete construction about 50 years ago, and its utilization for that purpose has since become increasingly widespread. It is generally accepted that the proportion of Portland cement replaced by the usual fly ash should not exceed about 20 percent to avoid significant reduction in the compressive strength of the resultant concrete, although some more cautious jurisdictions may impose lower limits, e.g., the 15 percent maximum authorized by the Virginia Department of Highways and Transportation (VDHT). As described in Lea on page 437, the substitution of fly ash tends to retard the early rate of hardening of the concrete so that the concrete exhibits up to a 30 percent lower strength after seven days testing and up to a 25 percent lower strength after 28 days of testing, but in time the strength levels normalize at replacement levels up to 20 percent. Increasing the substitution quantity up to 30 percent gives more drastic reduction in the early compression values as well as an ultimate strength reduction of at least about 15 percent after one year.
The limited substitution of fly ash for Portland cement in concrete formulations has other effects beyond compressive strength changes, both positive and negative. The fly ash tends to increase the workability of the cement mix and is recognized as desirably reducing the reactivity of the Portland cement with so-called reactive aggregates. On the other hand, fly ash contains a minor content of uncombusted carbon that acts to absorb air entrained in the concrete. Because entrained air desirably increases the resistance of the hardened concrete to freezing, such reduction in entrained air is undesirable, but can be compensated for by the inclusion as an additive of so-called air-entraining agents.
Therefore, there remains a need for compositions containing cement and fly and/or bottom ash, which can achieve superior strength when compared to cement alone.